Dow 10,000? Slam dunk. Dow 11,000? Not so much.

We last saw that level (on the way up, that is) six years ago, and we first crossed Dow 10,000 a full 10 years ago. (The latter fact is no doubt deeply depressing to buy-and-hold investors, but at least it has a nice symmetry to it.)

So here we are oh-so-close to regaining the psychologically significant Dow 10K, and yet in so many ways, it feels too good to be true. You needn't be a full-on bear to figure that this current rally is getting a little long in the tooth. The Dow's up 50 percent from the March bottom, the broader S&P 500 has gained nearly 60 percent since that time and the tech-heavy Nasdaq Composite has engorged itself by nearly 70 percent.

If you're a market participant who doesn't happen to be in a persistent vegetative state, you've got to be asking: How much longer can this go on?

Whether this is a massive bear-market-rally head fake or the beginning of a new bull run is a matter for psychics now and historians later. But we do know this: Even some of the more cautious and even bearish prognosticators and pundits see more short-term upside ahead, if for no other reason than the market is going up because the market wants -- nay, needs -- to go up, so that rank-and-file portfolio managers can keep their day jobs.

Fund managers, let's not forget, are held accountable to their benchmarks. Whether it's the S&P or (take a deep breath first) the MSCI U.S. Investable Market Consumer Discretionary Index (MSCICD), you've got to match or outperform your benchmark. After all, why would an investor pay extra fees for active management if his portfolio wrangler can't beat a low-cost index fund? That helps fuel momentum investing, where managers chase returns, buying high and selling higher, in order not to be left behind.

No, it's not an ideal investing situation, but it does bode well for further near-term gains. As Jeffrey Saut, chief investment strategist at Raymond James, told clients last week: "With many money management firms approaching their October year end, the performance pressure, subsequent bonus pressure, and ultimately career risk, continues to increase geometrically as the equity markets rally."

In other words, buy and buy some more -- or no bonus for you.

The good news is that this state of affairs could continue right up until the end of December, says Keith Hembre, chief economist at U.S. Bancorp's (USB) FAF Advisors in Minneapolis. "Our view is that you probably have a fair number of managers trailing their benchmarks, chasing returns," says Hembre. "And that will probably continue to year-end so they can have window dressing on their numbers."

Also supporting the argument for further shorter term gains is the fact that September -- ordinarily the cruelest month for equities -- has thus far been so strong, says Brett D'Arcy, director of investment and research at CBIZ Financial Solutions (CBZ).

"[This month's] performance is going to provide a lot of momentum," D'Arcy says by way of email. "To have the historically worst month be this good is going to get a lot of attention from the 'fence sitting' cash holders."

If that does indeed come to pass, we could be in for quite another momentum move up. According to Wood Asset Management, there's still enough money sitting on the sidelines to buy out the entire S&P 500.

The tough decision investors face is whether to book gains now -- and risk missing out on additional upside; or stay fully invested -- and risk losing hard-won paper gains in a pullback, correction or rout. Let's not forget that the economy is looking to have a pretty tepid recovery and there's still "way too much risk" in the equity markets, according to the presciently bearish David Rosenberg.

"Rosie," who left Merrill Lynch after the Bank of America (BAC) deal to become chief economist at Gluskin Sheff, told clients Friday that never before has the S&P 500 rallied 60 percent from a low in such a short time frame. Furthermore, the market has never rallied 60 percent at the same time the economy lost 2.5 million jobs.

"What is normal is that we see more than 2 million jobs being created during a rally as large as this," Rosie wrote. He believes the buying that's driving the market higher is most likely a combination of program trading, short covering and "portfolio managers desperately trying to make up for last years losses."

So what's a poor, lone retail investor to do?

Get a grip on those stop-loss orders and roll, roll, roll them up by the same percentage that your investments have jumped. If you bought a stock at, say, $50 with a stop-loss of $45, and now those shares have doubled to $100, make sure you double that stop-loss up to $90, too.

True, you could get stopped out of a position, but this will force you to sell higher and avoid getting killed if some unforeseen news or data snuffs out the current momentum run. It just might make you sleep better at night, too.